Sharks in Decline

We all know sharks are in trouble. We talk about the dramatic declines in shark populations, a result of high catch rates for shark fin, and as bycatch in fisheries targeted at other species. But what is the data these statements are based on? How are such declines calculated?

Like many researchers who study sharks, I frequently give seminars where I talk about declining shark numbers. Recently I went back to review much of the research literature supporting the decline in shark numbers, and I’ll highlight two of the most compelling articles here.

In the first report, Baum et al took a historical approach to estimate the change in numbers of large pelagic sharks by compiling data from fisheries logbooks. Specifically, they looked at the logs of the US tuna and swordfish longline fishing fleet in the Northwest Atlantic, from 1986-2000. While sharks are not targeted by this fishery, longline hooks catch many sharks who attempt to eat either the bait on the hook, or the captured prey. Each fishing boat records the number and species of the sharks they catch, and these numbers give an ongoing estimate of the abundance of each shark species in the region. The calculations of Baum et al found that 8 species of shark studied have declined by more than 50% in the past 15 years, with scalloped hammerheads (Sphyrna lewini) decreasing by 89%, thresher sharks (Alopias vulpinus and Alopias superciliosus) by 80% and white sharks (Carcharodon carcharias) by 79%.

What about coastal reef shark species, are they doing any better? In a separate article Robbins et al analyzed numbers of two common coastal sharks off the Australian Great Barrier Reef, whitetip reef sharks (Triaenodon obesus) and grey reef sharks (Carcharhinus amblyrhynchos). They counted sharks in areas with active fishing, in “no take” marine reserves, where boats are permitted but fishing is not allowed, and in “no go” marine protected areas, where boats are not allowed to enter. As a control for an undisturbed shark population – difficult to find these days – they counted sharks at the remote Cocos Islands (the Indian Ocean Cocos, not the Pacific Cocos), where there is little or no fishing.

Robbins et al found that “no go” areas carried far higher numbers of sharks than areas without this protection – close to the numbers of sharks seen at Cocos Island. Regions that were not entirely closed to boat traffic, however, the “no take” zones, showed 80% fewer whitetip reef sharks and 97% fewer grey reef sharks, even with fishing restrictions in place. Restricted fishing reserves thus appear to offer little protection for sharks, at least in this area, probably because enforcement is difficult, and much illegal fishing occurs. The authors estimated annual changes in populations of these two species, showing declines of 7% per year for whitetip reef sharks and 17% per year for grey reef sharks – rates that indicate likely extirpation (regional extinction) of these species within 20 years.

Think these statistics are depressing? Keep in mind that these studies are 5-8 years old, and there is little evidence that the status of sharks has improved since then. Recent victories in restricting shark fishing and regulating the fin trade are essential to prevent extinction of many shark species, but it will take a long time for these actions to impact such depleted populations.